1. Field of the Invention
The present invention relates to a method of preparing a copolymer and using such as a dispersing agent for titanate-based ceramic colloids.
2. Description of the Related Art
Titanate-based ceramic powder such as barium titanate and lead zirconate titanate are used extensively in the electronic industry. For example, barium titanate is used as a raw material for manufacturing capacitors, inductors, and thermistors, etc. In a manufacturing process, it is necessary to add a solvent to the titanate-based ceramic powder to produce colloids. At present, there are two types of solvents used by the industry, and they are water and organic solvents. Since the aqueous barium titanate colloid causes insignificant pollutions, therefore it becomes increasingly popular.
The dispersing effect of a titanate-based ceramic powder in a solvent has significant influence on the manufacturing quality of a product, and ceramic powder with a poor dispersing property will be piled up unevenly and produce a green sheet with lesser density, and the sintered part is less denser and easier to break, and thus resulting a low yield rate and poor electric properties. (Refer to Kumar V, “Solution-precipitation of fine powders of barium titanate and strontium titanate”, J. Am. Ceram. Soc., 82(10): 2580-84 1991). Therefore, industrial titanate-based ceramic colloids require an addition of a dispersing agent to facilitate the later manufacturing process and produce high-quality products.
As to aqueous ceramic colloids, most of the present industrial dispersing agents used are acrylate-based polymers or copolymers such as ammonium polymethacrylate (Darvan C) or sodium polyacrylate. In recent years, many researches are conducted on the subjects of using Darvan C or acrylic polymer as a dispersing agent for barium titanate and other ceramic such as aluminum oxide, titanium dioxide, and zirconium oxide powders. (Refer to Zhao JL, Wang XH, Gui ZL, Li LT, “Dispersion of barium titanate with poly(acrylic acid-co-maleic acid) in aqueous media”, Ceram. Int. 30(7): 1985-88 2004; Saravanan L, Subramanian S, “Surface chemical studies on the competitive adsorption poly(ethylene glycol) and ammonium poly (methacrylate) onto alumina”, J. Colloid Int. Sci. 284(2): 363-377 2005; Shi YC, Wu YS, Li JG, Li GZ, “Surface and rheology characterization of NH(4) PAA-stabilized nanosized TiO2 suspensions, “J. Dispersion Sci. Tech. 24(5): 739-743 2003□ Paik U, Hackley VA, Lee J, Lee S ”, Effect of poly(acrylic acid) and poly(vinyl alcohol) on the solubility of colloidal BaTiO3 in an aqueous medium”J. Mater. Res. 18(5): 1266-74 2003; Liu DM, “Influence of dispersant on powders dispersion and properties of zirconia green compacts”, Ceram. mt. 26: 279-87 2000). Although results show that Darvan C or acrylic polymer provides a good dispersing effect, further improvements are needed. For example, a barium titanate colloid using Darvan C as a dispersant has a poor dispersing effect when a small quantity of Darvan C is added, and the barium titanate particles in the colloid will settle quickly, and thus the colloid cannot maintain a suspending state for a long time.
Further, if Darvan C is used as a dispersing agent for the barium titanate colloids, the quantity of dissolved barium ions in aqueous solutions will be increased. Excessive barium ions will be deposited on the surface of the ceramic powder during a drying process, and thus will adversely affect the electric properties as well as other properties of the sintered part. The dispersing agent of the present invention can overcome the foregoing shortcoming and evenly disperse the colloids.
The present electronic components tend to be developed with a smaller size and more powerful functions, and thus breakthroughs for the manufacturing processes are urgently needed, and one of the subjects is to achieve a better dispersing effect for ceramic powers having a very small particle diameter. Therefore, a dispersing agent plays a main role on the application of dispersing various different ceramic powders. The titanate-based ceramic colloid having a good dispersing effect is absolutely superior in manufacturing processes and provides a good yield rate for ceramic products as well as a higher capacitance which can further comply with market requirements and enhance competitiveness.
Most of the present dispersing agents used for ceramic powders are polymeric surfactants, and the dispersant in a colloid relies on its dissociation to produce ionic functional groups. After the dissociated dispersant is adsorbed onto the powder, the surface of the powder will carry the same electric charge that can produce an electrostatic repulsion for the dispersion. In another dispersing mechanism, there are many side chains of the adsorbed dispersing agent on the surface of particles, and the powder can be dispersed very well due to the steric hindrance (Refer to Napper D H, Polymeric Stabilization of Colloidal Dispersions, Academic Press: New York, 1983). In summation of the description above, the type and the quantity of both functional groups and side chains in the molecular structure of a polymeric dispersant are major factors that affect the dispersing effect of the dispersing agent.